1. Field of the Invention
This invention relates to an apparatus and for filtering air in a predetermined area. More specifically, the present invention is directed to a method and device used for filtering contaminant from a contaminated area in an efficient and safe manner.
2. Description of the Related Art
It is often desirable to filter the air in a closed room due to the presence of contaminants. These contaminants could have been caused by equipments leaks, chemical spills or any other situation where contaminants have entered the air. An example of such a setting is a nuclear power facility, where it is desirable to reduce or eliminate the spread of radioactive particles so that the amount of radioactive particles that workers are exposed to is well below preestablished limits.
A typical ventilation unit has a filter unit and a fan. A gauge is mounted across the filter unit for measuring the differential pressure. The filter unit contains two filters, a prefilter and a HEPA (high efficiency particulate air) filter. Most known units can also be supplied with a carbon filter. All ventilation units, regardless of capacity, are operated in the same manner. Flexible ducting is coupled to the intake of the filter housing and directed to the area that requires ventilation. The exhaust fan is then started and the unit is run until the room has been completely ventilated or the pressure gauge indicates a high reading, suggesting that the filter(s) need changing.
The known prior art units have several problems. No visual method is known for determining the amount of air flowing through the system. To determine the system air flow requires that velocity measurements be taken, which involves placing a measuring instrument into a potentially contaminated piece of duct work and contaminating the instrument.
As the ventilation system operates, the filters in the filter unit begin to filter out the contaminant in the air. More contaminants are filtered out as the system continues to run. The contaminant filtered out of the air and trapped on the filters cause the static pressure across the filters to increase, which increases the static pressure facing the exhaust fan. The total flow through the ventilation system is thus decreased. The higher the static pressure existing across the filters, the smaller the air flow. Therefore, it is necessary to perform continuous measurements of the velocity to determine the air flow.
Because there is only a single pressure gauge across the filter housing that normally houses two filters, the prefilter and HEPA filter, the gauge indicates the total increase in static pressure across both filters as the filters collect contaminants. HEPA filters are able to withstand large static pressures, between 12 and 16 inches water gauge, before rupturing. However, the prefilter should be changed when the pressure across it reaches 0.5 inches water gauge according to the filter specifications set forth by manufacturers. As the single pressure gauge fails to differentiate between filters and does not indicate which filter is receiving the heaviest loading, the prefilter could easily rupture, which in turn damages the HEPA filter. Rather than allowing the prefilter to rupture, it is widely accepted practice to replace both filters each time the gauge has a high reading. This is wasteful of time and money and exposes employees to contaminants more often than is necessary.
Further, the differential pressure gauge across the filter unit does not present an accurate reading of the true differential pressure loading of the filters. Indeed, as the filters traps contaminants, the static pressure across the filters rises as well. Because of the increased static pressure across the filters, the static pressure facing the exhaust fan also increases, thus reducing the total system flow resulting in a reduced air volume flowing across the filters. As the filters have less air flowing across the gauge than they did originally, the static pressure reading is not a true reading.
These ventilation systems often are designed to be portable and, as noted above, have flexible ducting connected thereto that are directed to the area to be ventilated. This flexible ducting crushes or deforms easily, and if such should happen, the system flow will be considerably reduced.
Yet a further deficiency known with respect to prior models of ventilation units is that some models claim that the units includes a bag-in, bag-out filter housing that allows personnel to change filters inside a filter change bag, eliminating the need for respirators or the potential to spread contamination. In actuality, it is necessary to worry about contamination and have employees wear respirators, as the internal parts of the ventilation units are exposed.
U.S. Pat. Nos. 4,450,964 to Wood and 4,521,234 to Peebles, Jr. et al. are both directed at housings for filter units, and not an entire ventilation system. These housings are well known and commonly designed to be used as housings in ventilation systems.
U.S. Pat. No. 4,786,295 to Newman et al. teaches a filtration device for airborne contaminants. This device has many of the above-described problems. The main problem with this device is the lack of a gauge across each filter. Thus, it will be impossible to determine which filter needs to be replaced, and the possibility of rupture of the prefilter is always present.